The present invention relates to throttle and choke control mechanisms of carburetors for internal combustion engines, and more particularly to such a mechanism incorporating a choke-throttle, cold-start-setting latch mechanism that automatically positions the throttle valve slightly open when the choke valve is fully closed.
In small carburetors designed for use with low displacement gasoline fueled engines, such as used on chain saws, weed whips, lawn mowers, garden tractors and other small lawn, garden, and forestry portable appliances, manually operated choke and throttle controls are typical provided and often hand cranking is employed for starting the engine. Prior to the late 1970s, chain saws equipped with such choke and throttle controls often involved a basic starting sequence which left much to be desired. First the choke valve was fully closed to its start position, and then the starter rope was pulled until the engine fired. The closed choke valve usually caused the engine to immediately die at this first firing due to over-enrichment of the air/fuel (A/F) mixture. This is commonly referred to as a false start. At this point the choke valve had to be opened. Then the starter rope was pulled again until the engine finally began running.
This starting sequence was subsequently improved by adding another start-up control to the chain saw whereby the throttle valve could be held at a partly opened position, known as fast idle position. This generally avoided false starts due to the increased air flow permitted past the throttle valve.
In order to avoid the need for three separate manually operated controls, namely, a throttle control, a choke control and fast idle start control Johansson, U.S. Pat. No. 4,123,480, issued Oct. 31, 1978 (which is incorporated herein by reference), disclosed an improved chain saw engine control mechanism. In the ""480 patent a fast idle secondary lever 9 is pivoted on the choke valve shaft 11 and is operable to engage a latch arm of a throttle lever 4 fixed on the throttle valve shaft 2 to cause the throttle valve 1 to open to a predetermined angle corresponding to the fast idle position (FIG. 3). With this arrangement, the operator need only operate a single start-up control, namely the choke valve control (not shown) coupled to the choke shaft control lever 12 in order to set the throttle 1 in fast idle condition. Thus, when the operator moves the choke control to swing the choke valve 10 from fully open position (FIG. 1) to its fully closed start position (FIG. 3), the pivotal motion of choke shaft control lever 12, via a push coupling tang 14 on the adjacent fast idle lever 9, pivots fast idle lever 9 and causes its notch 8 to latch engage the throttle lever latch arm tang 7, thereby automatically setting the fast idle latch mechanism. The normal biasing forces exerted by the respective fast idle lever spring and throttle shaft return spring (i.e., biasing the fast idle lever toward push coupling with the choke lever: biasing throttle valve 34 toward closed) and also used to provide the latch closing forces.
Then, due to this automatic latch up, if the chain saw engine experiences a false start, the choke lever 12 may be moved to the open position (FIG. 4) without thereby moving the fast idle lever i.e., because it remains engaged with the throttle lever to retain the throttle valve 1 in the fast idle position. Once the chain saw engine starts, the operator simply depresses the throttle control trigger 6 to open the throttle valve 1. This pivots the throttle shaft lever 4, thereby causing it to disengage the fast idle lever 9 and thus cause release of the latch. If the choke valve 10 was still in the closed position at this point, the choke biasing spring 15, acting through the fast idle lever 9 and tang 14 coupling it to the choke lever, would automatically cause the choke valve 10 to be returned to full open position upon such unlatching of the fast idle lever 9 from the throttle lever 4 (FIG. 1).
One of the disadvantages of this fast idle starting system (FISS) ""480 patent design was its failure in practice when mass produced to insure complete and/or consistent closure of the choke valve 10 when setting the fast idle latch starting system. The specific problem has been found to be due to a pull-back or rock-back effect by the fast idle lever exerted on the choke lever resulting in the choke valve sometimes not being completely closed even though the operator has fully engaged the choke control to indicated start position. Further, it has been found that this problem is due to the need to provide an xe2x80x9cover-travelxe2x80x9d gap in the resting engagement of throttle lever tang in the fast idle lever notch to accommodate a stack up of normal manufacturing tolerances in the parts as manufactured for assembly into the fast idle latch mechanism.
Such manufacturing tolerances are, of course, necessary to set up minimum dimensional range limits or allowances to accommodate normal manufacturing equipment capabilities at acceptable manufacturing cost levels. This is a particular problem in producing carburetors for engines for chain saws, lawn mowers, clearing saws, weed whips, etc. that require very low manufacturing cost due to the low retail price of such consumer products. The problem is compounded due to the small size of the carburetors for such small engines, and the corresponding minuscule size of the choke and throttle parts involved in the carburetor mechanisms. These factors make it particularly difficult to reduce manufacturing tolerance allowances in order to reduce the adverse effects of unavoidable manufacturing dimensional variations in such tiny parts when assembled for operation in the mechanism.
Thus, in the case of the incomplete and/or inconsistent closure of the choke valve in the operation of the fast idle starting system of the ""480 patent arrangement, it has been found that, without the aforementioned over-travel gap allowance, a shift in tolerances for all parts (tolerance stack-up) in the latch mechanism to one end limit will render the choke valve incapable of reaching the fully closed position. This prevents, or at least hinders engine starting. On the other hand, and without such gap allowance, a tolerance shift in all of these parts to the opposite end limit will cause the fast idle lever to fail to even engage with the throttle lever, so that no xe2x80x9clatch upxe2x80x9d action occurs. This results in a loss of function of the entire choke throttle fast idle system.
The culprit in this resultant choke valve pull-back or rock-back problem has been found to be the push coupling of choke lever 12 with the fast idle lever 9 (via tang 14). This dictates that the actual latch-set position of choke valve 10 when initially swung to fully closed position will be controlled by the final latched-up position of fast idle lever 9. The over-travel gap in the engaged tang and notch parts allows the fast idle lever and throttle lever (if indeed engaged) both to be swung slightly back by their biasing springs until latched into their spring held, stable, latched position after manipulating forces are removed from the manual controls of the appliance. This problem of the adverse xe2x80x9cspring-backxe2x80x9d or xe2x80x9cpull-backxe2x80x9d effect on the fast idle start settings of the choke and throttle valves when latched will be further explained and seen in more detail hereinafter. Another prior art solution to the problem of achieving automatic fast idle setting of the throttle valve is found in Hermle U.S. Pat. No. 5,200,118, issued Apr. 6, 1993 and assigned to Walbro Corporation of Cass City, Mich., assignee of record herein. (U.S. Pat. No. 5,200,118 also being incorporated herein by reference). The ""480 patent is also described in the ""118 patent. It will be seen from FIGS. 1-5 of the ""118 patent, and by reference to the specification and claims of the ""118 patent, that the choke valve 10 is xe2x80x9cdivorcedxe2x80x9d as to its operator control handle 16 and associated linkage from the control handle 28 and associated linkage for the fast idle lever 20, which is thus independently operated through its own crank arm 24 of its bell crank 20. The ""118 system thus avoids the xe2x80x9cspring-backxe2x80x9d problem by adding a separate manual control 16 to operate the choke valve 10, and likewise the fast idle latch lever 20 is operated solely by actuating its own control member 28. It will be seen that with the ""118 patent system there is no tang coupling between choke lever arm 12 and the fast idle latch bell crank 20. Hence the ""118 patent system, although more complex in structure and mode of operation, does not present the aforementioned incomplete choke closure problem of the ""480 patent system.
Thus, the aforementioned prior art ""118 and ""480 patents neither address the problems nor provide a solution thereto that insures that, in the case of the ""480 type fast idle start mechanism, as manufactured in mass production practice, the choke will be able to reach the fully closed position at fast idle latch-up. Therefore, the problems of poor starting, or in worst case, xe2x80x9cno startingxe2x80x9d, continued to prevail for many years despite the wide spread use of the""480 system on carburetors supplied by several major carburetor manufacturers utilizing the ""480 system.
One recently commercially adopted solution to the foregoing problems is that set forth in Van Allen U.S. Pat. No. 6,000,683 issued Dec. 14, 1999 and also assigned to Walbro Corporation, which is incorporated in toto herein by reference. This ""683 patent invention works well when the choke valve completely closes and the fast idle lever has no play in the nested (locked-up) position. In this invention the small advancement from tooth to tooth may absorb some over-travel. Over-travel may thus be reduced due to the possibility to advance the fast idle lever one more tooth. However, due to part variability, the advancement from tooth to tooth may not be smaller than the over-travel, and hence the choke valve can in such cases still be pulled off full choke for such over-travel, albeit a small amount.
Still another recent solution to the foregoing over-travel and resultant choke valve pull-back, slight re-opening problem is provided by the invention disclosed and claimed in co-pending Pattullo U.S. patent application Ser. No. 09/252,257 filed Feb. 18, 1999, now U.S. Pat. No. 6,202,989, also assigned to Walbro Corporation and incorporated herein in toto by reference. The Pattullo application invention utilizes a fast idle lever and throttle lever in the carburetor automatic fast idle control mechanism similar to those of the aforementioned ""480 patent. However, in one preferred but exemplary embodiment disclosed in the Pattullo application, the choke shaft is made from a torsionally flexible material, such as Delrin(copyright) acetal plastic, that can be torsionally stressed to enable continued rotation of the choke shaft portion carrying the fast idle lever after the choke valve reaches full closure. Hence further pivotal motion of the fast idle lever past its choke closed position is produced before the fast idle lever reaches latch-up engagement with the throttle lever.
A novel spring biased, lost motion operating linkage for the choke valve and fast idle lever is thus achieved that prevents retrograde opening motion of the choke valve from its fully closed design position upon release of operator actuating force. This is achieved regardless of variations in the angular range of relative orientation of the fast idle lever free end with respect to the tang of the throttle lever throughout the range of tolerance stack-up positions of these parts, as well as the tolerance stack-up in the remaining operably cooperative mechanism parts when mass produced to the pre-existing tolerance specifications. The override capability of the choke shaft thus insures complete choke valve closure without concern for the required manufacturing tolerances.
Thus, the Pattullo application invention involving the aforementioned flexible choke shaft design achieves the goal of eliminating xe2x80x9cover-travelxe2x80x9d, because the choke valve closes well in advance of the fast idle lever and throttle lever nesting in lock-up. However, to nest these two levers the operator must twist the choke shaft via the choke lever. If the operator does not twist the choke lever far enough, the two levers will not nest. Hence, the control linkage to operate the choke lever must insure that sufficient choke shaft twisting is achieved by the time the linkage reaches its setting for fast idle start.
Another limitation of this Pattullo system is that the choke shaft must be made of a flexible material, such as the plastic material specified in the Pattullo application, for this design to function properly. Moreover, because the choke shaft must twist, the choke lever must be located on the same side of the carburetor as the fast idle lever. That is, if the choke lever and fast idle lever are mounted on opposite sides of the carburetor, the choke shaft twisting action will not transmit all the way through the choke shaft due to the choke valve plate being inserted through the choke shaft and thereby rigidifying the same against twisting, i.e., the twisting stops at the choke valve plate. Thus, there is a need for further improvements in fast idle starting systems that will overcome these limitations of the Pattullo FISS structure and mode of operation as well as being applicable to carburetors with non-twistable choke shafts, and that will also overcome the aforementioned limitations of the Van Allen ""683 patent improvements.
Another prior art structure added to many carburetor choke linkages are ball and spring detents that are operable to apply a force to help keep the choke valve closed. However, these detent systems add cost, and in any event are not easily used in conjunction with a FISS because they do not generate enough force to overcome the rock-back forces produced by the powerful throttle valve spring.
Accordingly, among the objects of the invention are to provide an improved carburetor choke and throttle mechanism providing automatic throttle fast idle setting capability that obtains the advantages of the Johansson U.S. Pat. No. 4,123,480 system as compared to the alternative system of the Hermle U.S. Pat. No. 5,200,118, while at the same time overcoming the aforementioned problems encountered in mass production of carburetors employing the ""480 patent system so that when the parts are made to the existing entire range of dimensional tolerances the fast idle lever will nevertheless properly engage the throttle lever in such a manner that the choke valve plate will move to, and remain in, the fully closed position, thereby eliminating the poor starting or worse case, no starting, conditions described hereinabove.
Another object of the invention is to provide an improved carburetor choke and throttle automatic fast idle mechanism of the above character which solves the aforementioned problems by replacing a minimal number of parts with an improved fast idle lever that can be used in a conventional FISS configuration or with the improved torsionally resilient choke shaft and choke valve plate subassembly of the aforementioned Pattullo co-pending application, at less cost than that of the replaced parts, and one that can be substituted as a running change in production, that does not significantly alter the manufacturing and assembly processes already employed in the manufacture of the prior mechanism, which is readily retrofitable to existing carburetors as a field repair item if desired, and which does not require any tightening up of existing manufacturing tolerances and thus avoids the additional costs of attempting to achieve such improved precision in processing methods and machinery as well as assembly equipment and fixturing.
A further object of the invention is to provide an improved FISS mechanism of the above character which is readily adaptable for use with a choke shaft that is metal and thus torsionally rigid, as well as with a plastic choke shaft that is torsionally resilient and twistable in its mode of operation as in the aforementioned Pattullo application system, which provides the option of eliminating ball and spring detents that have been used to help the choke valve stay completely closed, and which is adaptable to so-called xe2x80x9csplit linkagexe2x80x9d carburetors having the choke lever and fast idle levers disposed one on each of the opposite sides of the carburetor from each other, which insures that the throttle lever and fast idle lever are rendered operably independent from the choke lever in the fast idle starting condition with the choke closed to thereby eliminate the choke valve pull-back effect, which insures that the throttle valve fast idle position is held with more accuracy and which insures that manufacturing tolerance stack-up cannot adversely affect choke valve closure even with simple lever configurations, thereby allowing for complete closure of the choke valve when the fast idle lever is engaged while preventing interference with the choke lever from the movement or positioning of the fast idle lever when nestably locking up with the throttle lever in establishing the fast idle start condition.
Still another object is to provide an improved fast idle starting system of the aforementioned character that will insure complete and consistent closure of the choke valve on fast idle starting systems for diaphragm carburetors, which prevents the choke valve from floating and/or springing-back so as to prevent inconsistent closure of the choke valve from these effects, which is of lower cost and more forgiving to tolerance stack-up than current ball and spring detent systems, and which is better suited to the xe2x80x9cflexible shaftxe2x80x9d fast idle starting systems of the aforementioned Pattullo co-pending application.
In general, and by way of summary description and not by way of limitation, the invention fulfills the foregoing objects by merely substituting a novel fast idle lever for the corresponding prior art part, the remaining choke shaft, choke valve plate and throttle lever parts of the carburetor automatic fast idle control mechanism being retained and utilized without change, if desired.
In one preferred but exemplary embodiment utilizing the aforementioned Pattullo flexible shaft feature, the choke shaft is made from a torsionally flexible material, such as Delrin(copyright) acetal plastic, that can be torsionally stressed to enable continued rotation of the shaft portion carrying the fast idle lever after the choke valve reaches fall closure. This then produces further pivotal motion of the fast idle lever before it reaches latch-up engagement with the throttle lever.
Additionally or alternatively, the choke lever carries a resiliently flexible latch hook that is operable to resiliently pull the choke valve fully closed. This hook releases when the choke is moved by operator control from closed toward open position while the fast idle lever remains latched at engine start-up. The hook re-latches when the fast idle lever is released from lock-up with the throttle lever. Thus, an improved spring biased, lost motion operating linkage for the choke valve and fast idle lever is achieved in a simple, low-cost manner that prevents retrograde opening motion of the choke valve from its fully closed design position upon release of operator actuating force. This is achieved regardless of variations in the angular range of relative orientation of the fast idle lever free end with respect to the tang of the throttle lever i.e., throughout the range of tolerance stack-up positions of these parts, as well as that of the remaining operably cooperative mechanism parts when mass produced to the pre-existing tolerance specifications. The override capability of the choke shaft thus insures complete choke valve closure. without concern for the required manufacturing tolerances.
As a common and primary feature to both twistable and non-twistable choke shaft embodiments incorporating the invention, the distal free edge surface of the fast idle lever blade that is engaged by the tang of the throttle lever during fast idle latch-up is modified so that initially the tang exerts a resistive torque, and then just prior to such latch-up engagement a momentary additive torque is developed in the fast idle lever acting in the same rotational direction as the propelling torque applied by manual rotation of the choke lever. This camming interengagement accelerates fast idle lever rotation relative to choke lever rotation and thereby opens up a leading gap so that there no longer is push contact between the choke lever finger and fast idle lever tang. This additive torque is developed by a camming action of the throttle lever tang as its powerfill biasing spring causes the tang to slide down a camming ramp surface of the fast idle lever blade distal edge toward a lock-up xe2x80x9cV-notchxe2x80x9d therein. This xe2x80x9cV-notchxe2x80x9d is located by design so that when the throttle lever tang engages the same to latch and thereby hold the fast idle lever immobile, the leading gap, albeit smaller, is still present between the fast idle lever tang and the pusher finger of the choke lever. Hence, should counter-rotation of the fast idle lever occur, it is stopped by latch-up action before such counter-rotation can produce a push-back effect on the choke lever. Hence, spring-back or pullback re-opening the closed choke valve cannot occur.